There are many applications in which it would be desirable to employ brittle high-modulus yarns, that is, elongated multifilamentary members formed of materials exhibiting high ratios of stress to strain, but exhibiting brittleness making them difficult to handle. Such materials include relatively well-known materials such as carbon fiber and fiberglass, and less common materials such as basalt, quartz, and boron. While these relatively brittle materials are used successfully in a variety of applications, their brittleness has prevented them from being used in certain manufacturing processes which are desirable.
In particular, there are many applications in which it is helpful to provide a yarn with a “tacky” (that is, somewhat adhesive) coating, so that the yarn can be disposed in a desired position against a substrate in manufacture of a product, followed by application of heat and pressure, securing the yarn in position for further processing.
For example, in manufacture of laminated products such as sails, yarns are laminated between opposing membranes. The opposed membranes are bonded to one another, encapsulating and capturing the yarns. The yarns then provide the required tensile strength, while the membranes provide air-tightness. See, e.g., Conrad U.S. Pat. No. 4,708,080. This is a popular way to make sails, because the tensile strength characteristics of the sail can be tailored to the anticipated loads by careful disposition of the yarns.
However, due to the brittleness of the high-modulus materials mentioned, this process cannot be practiced as readily with these as with less brittle yarns; it would be desirable to adapt the high-modulus materials so that they could be used as are the less brittle (but lower modulus) yarns.
More specifically, as typically practiced, the manufacture of laminated sails is begun by disposing a panel of the membrane material (typically Mylar polyester) over a table shaped to the desired curvature of the sail. Alternatively, as discussed in the Conrad patent, the material can be laminated on flat tables, and “broadseamed”, i.e., adjacent panels are joined along curved seams, to define the desired shape of the sail.
In either case, yarns of the desired material are disposed in desired patterns over the membrane, corresponding to the anticipated loads. Certain less brittle yarns, such as “Kevlar” aramid, can be provided with a tacky adhesive coating, so that when the yarns are urged into contact with the membrane, typically with a heated roller applying heat and pressure, they will hold their position. A second membrane of the desired material is then disposed over this assembly, and the whole laminated together, typically by application of heat and pressure. (Those of skill in the art will recognize that this is a very simplified description of the process, and in particular that various additional layers may be incorporated into the basic structure.)
However, certain brittle high-modulus materials that would be desirably used as yarns in the above process (and many other processes) cannot be treated as above. In particular, if yarns of the relatively brittle carbon fiber, fiberglass, basalt, quartz and/or other brittle inorganic materials are coated with a tacky adhesive and the yarn is wound onto a spool for shipment and subsequent processing, the adjacent strands of yarn on the spool will tend to bond to one another, a problem known as “blocking” in the industry.
More specifically, where sections of the yarn contact one another as they are wrapped around the spool, the tacky adhesive on the yarn tends to bond the juxtaposed sections to one another. Subsequently, as the yarn is paid off the spool, substantial force is exerted between individual filaments of the sections of the yarn in contact with one another; this force leads to an unacceptable degree of breakage of the brittle individual filaments and loss of strength of the yarn. To avoid this “blocking” problem, the multiple filaments making up yarns made of these brittle materials are typically adhered to one another (for stability in handling) by a coating of a drying, non-tacky adhesive. In order that the yarn can be secured in a desired position on the membrane, it is passed through a bath of a tacky adhesive just prior to being urged against the membrane with application of heat and pressure, which ensures that it will stay in place until the entire surface of the membrane has had yarns applied in the desired pattern. However, this additional processing step adds complexity, cost, and weight; it would be preferred to provide yarns of the desired high-modulus materials having a tacky coating to allow simpler processing.
Alternatively, the yarn can be coated first with a high-tack adhesive followed by a controlled layer of release agent; however, the amount of release agent must be controlled carefully to ensure that it does not interfere with formation of a high-integrity bond in the final product. More specifically, blocking can still present difficulty if insufficient release agent is applied, while the presence of excessive release agent adversely affects the ultimate bond. Accordingly, the release agent is desirably avoided completely.
One apparent solution to this problem would be to apply an adhesive coating to the yarn of a material that is not tacky until heat and pressure are applied, so as to avoid “blocking”; unfortunately, no suitably compatible adhesive is known. In particular, all known adhesives which do not tend to self-adhere, e.g., as a coated yarn is spooled, do not develop sufficiently strong bonds when heated and urged into contact with a substrate coated with a similar adhesive.
More specifically, a bond of great integrity is required between the yarns and the films, and the films to one another, to provide adequate service life to the sail. Applicant's testing indicates that polyester adhesive coatings are desirably applied to the films and to the yarns, although, as discussed below, they are preferably not the same polyester adhesive. More specifically, this testing indicates that the best bonding is achieved through use of a more tacky adhesive on the yarns, e.g., a “Vitel” co-polyester adhesive from Bostik Adhesives, and a non-tacky polyester adhesive on the films. However, use of the selected tacky Vitel adhesive on brittle yarns leads to the “blocking” problem. Other non-tacky adhesives can be used, e.g., low-tack ethylene vinyl acetate (EVA) can be used as the adhesive on the films and on the yarns, but this does not result in as strong a bond.
For similar reasons, brittle, high-modulus yarns often cannot be used as desired in various additional manufacturing processes involving one or more of weaving, knitting, braiding, filament winding, and laminating steps, or in manufacture of “laid-up” products, such as non-woven fabrics known as “scrims”.